The polymer electrolyte fuel cell generally has a structure, in which a plurality of single cells exerting a power generation function is stacked. Each of the single cells has a polymer electrolyte membrane and a membrane electrode assembly (MEA). The MEA includes a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane from an anode side and a cathode side. Examples of the polymer electrolyte membranes include Nafion™ (manufactured by Du Pont). Further, each of the single cells may additionally have a membrane electrode gas diffusion layer assembly that includes a pair of gas diffusion layers (GDL) for sandwiching the membrane electrode assembly and dispersing supply gas on each of the anode side and the cathode side. Then, the membrane electrode assembly and the membrane electrode gas diffusion layer assembly owned by an individual single cell are electrically connected to the membrane electrode assembly and the membrane electrode gas diffusion layer assembly of another neighboring single cell via a separator. By the stacking and connection of the single cells in this way, a fuel cell stack is constituted. The fuel cell stack can function as a power generator usable for various applications.
A power generation mechanism of a polymer electrolyte fuel cell will be explained briefly. In the operation of the polymer electrolyte fuel cell, fuel gas such as hydrogen gas is supplied to the anode side of the single cell, and oxidant gas such as air or oxygen is supplied to the cathode side. As the result, in each of the anode and the cathode, electrochemical reactions shown by the following reaction formulae (1) and (2), progress to generate electricity.H2→2H++2e−  (1)2H++2e−+(1/2)O2→H2O  (2)
As a catalyst component for accelerating electrochemical reactions shown by the above-mentioned reaction formulae (1) or (2), generally, platinum is used.
As shown in Japanese Patent Application Laid-Open Publication No. 2009-295341, there is proposed a membrane electrode assembly for a purpose of improving durability of the membrane electrode assembly under the low temperature environment and low-temperature startability of a fuel cell. The membrane electrode assembly is a membrane electrode assembly in which a catalyst electrode is assembled on both sides of the electrolyte membrane, and at least one catalyst electrode includes a first and a second catalyst layer containing a catalyst, carbon and an ionomer. The weight ratio of the ionomer relative to the carbon in the second catalyst layer is from 0.4 to 0.75.